Light sensitive circuits are well known in the industry. Typically, such a circuit includes a light source such as a light emitting diode (LED) which directs light towards a light sensor. Light completes a circuit which enables (or disables) some control circuit until the light path is interrupted. In most instances, the light path is interrupted by introducing an opaque object between the LED and the light sensor.
Such apparatus is used for example to sense the presence of a credit card. A card in this instance, might be inserted in a slot in a manner to interrupt a light path between an LED and a light sensor. But LED's and light sensors in such apparatus are exposed to contaminants which coat the sensitive surfaces of the devices and thus degrade the ability of the devices to discriminate between the presence and absence of a card. Further, cards exhibit varying degrees of opacity. In view of the fact that the performance of the LED's and sensors also varies with time, it becomes clear that credit card readers including such LED's and sensors require considerable maintenance if they are to perform continually at commercially acceptable levels.